Apparatus for oxidative coupling of methane

ABSTRACT

An apparatus for oxidative coupling of methane having a coupling reactor in the shape of a vertical pipe, a gas-solid separator, a catalyst feedback system and, between the gas-solid separator and catalyst feedback system, a catalyst cooling system for cooling the catalyst.

This is a Division of application Ser. No. 08/564,556 filed on Nov. 21,1995, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing a hydrocarbonhaving a carbon number of at least 2, such as ethylene, by means ofoxidative coupling of methane, and to an apparatus to be used in themethod. In particular, the present invention relates to a method foroxidative coupling of methane, at an excellent conversion and thermalefficiency, which employs a relatively small-scale and simple apparatusfor producing a hydrocarbon having a carbon number of at least 2 such asethylene, which is useful as liquid fuel and material for variousorganic syntheses, from a methane source such as natural gas, as well asrelating to such an apparatus.

2. Background Art

In recent years, demand for natural gas as an energy resource or amaterial for chemical syntheses has been increasing. This is consideredto be due to the greater deposited amount of natural gas than that ofpetroleum, exhaustion of which is foreseen, and due to the smalldischarge amount of carbon dioxide gas from natural gas combustion perits calorific value, giving it a relatively small influence on globalwarming. In recent years, exploration activities have achieved manydiscoveries of natural gas deposits; however, there are many cases inwhich these discoveries have not proceeded to exploitation sinceexploitation according to conventional techniques requires a largeamount of plant and equipment investment, and since the infrastructure,such as pipelines, at the discovery areas are insufficient. Techniquesfor liquefaction of natural gas have been put to practical use, such asa technique of producing liquefied natural gas (LNG) at an extremely lowtemperature under a high pressure, and a technique of converting naturalgas into liquid hydrocarbon fuel via production of synthesis gasaccording to steam reforming; however, important problems in proceedingto exploitation of a gas field exist in the large amounts ofexploitation investment, such as plant and equipment investment andoperating costs, according to any of these conventional techniques.Therefore, in order to make effective use of natural gas which isdiscovered in a remote area, establishment of a technique for convertingnatural gas into liquid hydrocarbon fuel, which is more efficient andrequires less cost than conventional techniques, has been desired.

In order to make effective use of methane, a technique has been soughtafter for converting the entire amount of natural gas containing methaneinto liquid fuel or synthesis raw material, which has a boiling pointhigher than that of the natural gas, by means of a simple installationand method which can be employed at the place of natural gas production.

Methods for producing liquid fuel from natural gas have been hithertoknown such as an indirect method in which natural gas is converted intoa synthesis gas which contains H₂ and CO by a process such as steamreforming, and then liquid fuel such as methanol, gasoline, kerosene,gas oil or the like is produced by another process such as theFischer-Tropsch process; as well as a direct method in which methane,which is a main component of natural gas, is converted by a methaneoxidative coupling reaction into unsaturated lower hydrocarbons such asethylene, and then liquid fuel or the like is produced by apolymerization reaction. Among these methods, interest has been focusedon direct methods in recent years since an indirect method requires alarge-scale apparatus in the process of producing the synthesis gas,and, in addition, since indirect methods have complicated procedures dueto incorporation of processes such as the Fischer-Tropsch syntheticprocess.

Methane oxidative coupling (hereinafter simply referred to as"coupling") reaction is a reaction in which methane is allowed to reactwith oxygen in the presence of a catalyst at a temperature of at least500° C. so as to produce unsaturated lower hydrocarbons (mainlyethylene), saturated lower hydrocarbons (mainly ethane), water, and thelike, as represented by the following formula (1):

    CH.sub.4, O.sub.2 →C.sub.2 H.sub.6, C.sub.2 H.sub.4, H.sub.2 O (1)

Since a combustion reaction of methane also proceeds along with theabove reaction in this temperature range, a part of the methane isconsumed in this combustion reaction so as to produce carbon oxides,such as carbon oxide (CO) and carbon dioxide (CO₂), as by-products. Inaddition, since the coupling reaction and the combustion reaction areexothermic reactions which generate a large amount of heat, thetemperature in the system of the reaction abruptly rises. In a methanecoupling reaction in general, the reaction does not progresssufficiently at a temperature lower than 500° C., at which the activityof the catalyst is low; and the yield of product hydrocarbons decreasesat a temperature exceeding 900° C., at which the combustion reactionbecomes dominant. Therefore, the key is to establish an economicaldirect method to make effective use of the generated heat and theby-product carbon dioxides while maintaining the reaction temperaturebetween 500° C. and 900° C.

A coupling process which has been conducted in a conventional fixed-bedadiabatic reactor is not practical since the ratio of conversion intounsaturated hydrocarbons with a temperature increase in the rangebetween 350° C. and 400° C., which is the range within which thetemperature is permitted to increase, is between 10% and 15% at highest.A fluidized-bed reactor, which improves efficiency in heat exchange, isoperable at a higher methane conversion; however, a process using such afluidized-bed reactor requires great cost for installing a large-scalecooling system such as a huge cooling pipe, so as to remove a lot of thereaction heat. Furthermore, such a reactor alone cannot make effectiveuse of the generated heat and the by-product carbon oxides.

Methods which can solve the above problems have been proposed such as amethod in which saturated hydrocarbons of carbon numbers of at least 2are introduced into a coupling reactor together with methane, and thenthe saturated hydrocarbons are pyrolyzed using the large amount of heatgenerated in the methane oxidative coupling so as to obtain unsaturatedhydrocarbons and hydrogen. Moreover, there is a publication concerning atrial in which the hydrogen produced in the above reaction is allowed toreact in a methanation reactor with the carbon oxides which are producedas by-products in the system, so as to produce methane, which is fedback to the coupling reactor (see J. H. Edwards, K. T. Do, R. J. Tyler,1989 International Chemical Congress of Pacific Basin Societies,Honolulu, Dec. 17-22, No. 169).

Although these methods which have hitherto been proposed gave somesuggestions for efficient use of the large amount of generated heat andthe by-product carbon oxides, problems still remain. For example, in thecase where a methane oxidative coupling and a pyrolysis of saturatedhydrocarbons are carried out in the same reactor, it is extremelydifficult to control the temperature since the former reaction isexothermic while the latter is endothermic. In addition, a method forcontrolling the reaction temperature in this process has not yet beensuggested. Furthermore, with respect to the molar ratios of the carbonoxides and H₂ which are produced in this reaction, the amount of thecarbon oxides is excessive for methanation in the methanation reactor;therefore, in order to regulate the molar ratios, a methanation whichrequires burdensome installation and energy must be carried out, inwhich, for example, the entire amount of CO₂ produced in the reactionsystem is once separated and removed in a decarbonation process, andthen, to the residual gas which does not contain CO₂, CO₂ in astoichiometric ratio for the methanation reaction is added.

SUMMARY OF THE INVENTION

The present invention intends to solve the above problems; therefore,the object of the present invention is to provide a methane oxidativecoupling method and an apparatus therefor by which a hydrocarbon havinga carbon number of at least 2 (hereinafter referred to as "C₂₊ ") can beeconomically produced at a high yield by effectively controlling thetemperature in the reaction system using a simple method as well as bymaking effective use of the generated reaction heat and the by-productcarbon oxides.

The above object can be solved by providing a method for oxidativecoupling of methane which is characterized by comprising: a couplingstep, in which a coupling feed gas containing methane and a gascontaining oxygen are supplied to a conveying catalyst bed which isformed by an ascending gas stream which contains a catalyst, and thenmethane and oxygen are allowed to react so as to produce a couplingproduct gas; a catalyst separation step for separating the catalyst fromthe coupling product gas; and a catalyst feedback step for feeding backthe thus-separated catalyst to the coupling step.

According to the method for oxidative coupling of methane, since thecoupling feed gas and the gas containing oxygen are supplied to theconveying catalyst bed which is formed by an ascending stream whichcontains a catalyst, the reaction system does not accumulate excessiveheat, the combustion reaction as a side reaction is hindered, and theselectivity of the coupling reaction is enhanced. Furthermore, as theresult of the reduction in the temperature of the fed-back catalyst andthe smooth controllability of the temperature during the couplingreaction, the yield of the C₂₊ hydrocarbon is improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of an embodiment of the couplingapparatus according to the present invention.

FIG. 2 is a flow diagram of an embodiment of the coupling methodaccording to the present invention.

FIG. 3 is a flow diagram of another embodiment of the coupling methodaccording to the present invention.

FIG. 4 is a partial flow diagram of an embodiment of the coupling methodaccording to the present invention in which an LNG separation process isincorporated in the process shown in FIG. 2.

FIG. 5 is a partial flow diagram of an embodiment of the coupling methodaccording to the present invention in which an LNG separation process isincorporated in the process shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The conveying catalyst bed used in the coupling method of the presentinvention can be formed in a manner such that catalyst particles areentrained by at least a part of the coupling feed gas. When this is thecase, at least a part of the coupling feed gas can be supplied to anintermediate position of the conveying catalyst bed. The gas containingoxygen is supplied preferably to an intermediate position of theconveying catalyst bed.

The molar ratio of oxygen/methane which are supplied to the couplingstep is preferably between 0.05 and 0.5. The amount of the catalystwhich is supplied to the coupling step is preferably between 0.5% and 5%by volume with respect to the volume of the entirety of gas supplied tothe coupling step in the conveying catalyst bed. The reactiontemperature in the coupling step is maintained preferably between 750°C. and 900° C. The reaction pressure in the coupling step is maintainedpreferably between atmospheric pressure and 10 kg/cm² G. In addition,the residence time in the coupling reaction zone is preferably between0.1 to 10 seconds.

The catalyst which is separated in the catalyst separation step ispreferably cooled before being introduced to the catalyst feedback step.

Between the catalyst separation step and the catalyst feedback step, acracking step is preferably provided in which the separated catalyst isbrought into contact with a cracking feed gas which contains a saturatedhydrocarbon of C₂₊, and then a cracking reaction is allowed to occur soas to produce a cracking product gas which contains an unsaturatedhydrocarbon. In the cracking step, it is preferable that the catalyst isallowed to descend from the upstream so as to form a fluidized bed withthe cracking feed gas.

Since the catalyst which is separated from the coupling product gas isat a high temperature, the cracking reaction, which is endothermic,proceeds by bringing the catalyst into contact with the cracking feedgas without necessity of additional heat.

The coupling method of the present invention may further comprise: amixing step for mixing the coupling product gas and the cracking productgas so as to form a product gas mixture; a hydrocarbon separation stepfor separating and collecting a C₂₊ hydrocarbon from the product gasmixture leaving a residual gas, which is split into a bypass gas and abranch gas; a decarbonation step for removing carbon dioxide from thebranch gas leaving a decarbonated gas, which is brought into confluencewith the bypass gas so as to form a confluence gas; and a methanationstep for methanating the confluence gas so as to produce a methanatedgas; and at least a part of the methanated gas is fed back to thecoupling step as at least a part of the coupling feed gas.

By splitting the residual gas, which remains after theseparation-collection of the C₂₊ hydrocarbon from the coupling productgas, into the branch gas and the bypass gas, then removing carbondioxide only from the branch gas, then methanating the decarbonated gastogether with the bypass gas, and then feeding back the methanated gasto the coupling step, the burden to the decarbonation step is lightened,and thus the installation scale and the energy consumption for thedecarbonation step can be reduced.

Alternatively, it is possible that the residual gas is initiallymethanated into a methanated gas in the methanation step, and thencarbon dioxide which is contained in the methanated gas is removed inthe decarbonation step leaving a decarbonated gas, at least a part ofwhich is fed back to the coupling step as at least a part of thecoupling feed gas.

By methanating the residual gas which still contains excessive carbonoxides, the equilibrium of the methanation reaction shifts towardproduction of methane, and thus larger amounts of hydrogen and thecarbon oxides are converted into methane.

Furthermore, by methanating the residual gas, without splitting, so asto produce the methanated gas, then removing carbon dioxide from themethanated gas, and then feeding back at least a part of thedecarbonated gas to the coupling step, components other than methane inthe coupling feed gas are reduced, and thus the yield of the C₂₊hydrocarbon in the coupling step is improved.

Between the mixing step for mixing the coupling product gas and thecracking product gas and the hydrocarbon separation step for collectingthe C₂₊ hydrocarbon from the product gas mixture, a polymerization stepmay be provided for allowing an unsaturated hydrocarbon which iscontained in the product gas mixture to polymerize.

At least a part of the coupling feed gas may be introduced to thedecarbonation step so as to remove acidic impurities which are containedin the coupling feed gas. In the case where natural gas is used as thecoupling feed gas, it is possible that at least a part of the naturalgas is introduced to the decarbonation step so as to remove acidicimpurities which are contained in the natural gas, then a saturatedhydrocarbon of C₂₊ which is contained in the natural gas is removedtherefrom, then the thus-removed saturated hydrocarbon of C₂₊ isintroduced as the cracking feed gas to the cracking step, and then theresidual gas is introduced as the coupling feed gas to this feedbacksystem.

The present invention also provides an apparatus for producing acoupling product gas which contains an unsaturated hydrocarbon byallowing reaction gases, which include a coupling feed gas containingmethane and a gas containing oxygen, to react in the presence of acatalyst, characterized by comprising a coupling reactor in the shape ofa vertical pipe, a gas-solid separator, and a catalyst feedback means,which are connected in a loop; the coupling reactor comprising acatalyst-bed formation section, which is provided at a bottom portion ofthe coupling reactor, for forming a conveying catalyst bed which isformed by an ascending stream which contains the catalyst, a gasintroduction nozzle for supplying at least a part of at least one of thereaction gases to the conveying catalyst bed, a coupling reactionsection for producing a coupling product gas by allowing methane andoxygen to react in the presence of the catalyst, and acoupling-product-gas withdrawal outlet, which is provided at a topportion of the coupling reactor, for withdrawing the coupling productgas together with the catalyst; the gas-solid separator being providedfor separating the coupling product gas and the catalyst apart; and thecatalyst separation means being provided for feeding back thethus-separated catalyst to the catalyst-bed formation section of thecoupling reactor.

The gas introduction nozzle of the coupling reactor may be a double-tubenozzle for simultaneously introducing at least a part of the couplingfeed gas and at least a part of the gas containing oxygen. It ispreferable that the gas-solid separator is a cyclone. A catalyst coolingmeans for cooling the catalyst is preferably provided between thegas-solid separator and the catalyst feedback means. A cracking reactoris preferably provided between the gas-solid separator and the catalystfeedback means. It is preferable that the cracking reactor is providedfor forming a fluidized bed from the catalyst which is separated in thegas-solid separator and from a cracking feed gas which contains asaturated hydrocarbon of C₂₊, so as to bring the catalyst and thecracking feed gas into contact with each other, and allowing a crackingreaction to occur so as to produce a cracking product gas which containsan unsaturated hydrocarbon.

The present invention will be explained in detail in the following, byway of referring to the drawings.

FIG. 1 shows an embodiment of a coupling apparatus 60 to be used for thecoupling method, in which the coupling step and the cracking step arecombined, according to the present invention. In FIG. 1, the couplingapparatus 60 is constructed mainly from a loop in which a couplingreactor 10, a gas-solid-separator 20, a cracking reactor 30, and acatalyst feed back means 50 are connected; and a catalyst cooling means40.

The coupling reactor 10 is in the shape of a vertical pipe, whichcomprises a catalyst-bed formation section 11, which is provided at abottom portion of the coupling reactor 10, for forming a conveyingcatalyst bed which is formed by an ascending stream which contains thecatalyst, a gas introduction nozzle 12 for supplying a coupling feed gascontaining methane and/or a gas containing oxygen to the conveyingcatalyst bed, a coupling reaction section 13 for producing a couplingproduct gas by allowing methane and oxygen to react in the presence ofthe catalyst, and a coupling-product-gas withdrawal outlet 14, which isprovided at a top portion of the coupling reactor 10, for withdrawingthe coupling product gas together with the catalyst.

The construction of the catalyst-bed formation section 11 allows acatalyst-bed formation gas 1 to be injected thereinto via a lower end 15at a high speed so as to form the ascending stream, a catalyst 5 to besupplied from the catalyst feedback means 50 to an upper end 16 thereofso as to be mixed with the ascending stream, and the mixture to becarried upwards.

In this embodiment, the gas-solid separator 20 may be a cyclone, inwhich the catalyst 5 is separated from a coupling product gas 3. Thecatalyst 5 descends gravitationally from the cyclone 20 through acyclone dip leg 21, lower end of which is dipped within the fluidized beof catalyst in the cracking reactor 30 to a suitable depth.

The cracking reactor 30 is a chamber in the shape of a vertical columnwhich comprises a product-gas-mixture withdrawal outlet 31, which isprovided at a top portion, a ring-shaped distributor 32, which isprovided in the vicinity of a bottom portion, for introducing a crackingfeed gas, and a catalyst withdrawal outlet 33, which is provided at abottom portion. In addition, the cracking reactor 30 is provided with acooling means 40 comprising a coiled cooling pipe above the distributor32, or such a means as an exterior cooler to which some of the catalystis introduced to be cooled and from which the cooled catalyst isreturned to the reactor.

The catalyst which is withdrawn from a bottom portion of the crackingreactor 30 is carried from the catalyst withdrawal outlet 33 via thecatalyst feedback means 50, and fed back to the catalyst-bed formationsection 11 of the coupling reactor 10.

A coupling process can be carried out, for example, by using thecoupling apparatus 60 in FIG. 1.

First, a coupling feed gas 1 containing methane is injected from thelower end 15 into the coupling reactor 10 so as to form therein anascending stream of the coupling feed gas 1. By introducing a catalyst 5from an upper end 16 of the catalyst-bed formation section 11, thecatalyst 5 is mixed and dispersed in the ascending stream and carriedupwards. Then, by introducing a gas containing oxygen 2 from the gasintroduction nozzle 12, the coupling reaction occurs in the presence ofthe catalyst in the coupling reaction section 13 of the coupling reactor10, so as to produce a coupling product gas 3 which contains aunsaturated hydrocarbon. The thus-produced coupling product gas 3,accompanied by the catalyst 5, flows out from the coupling-product-gaswithdrawal outlet 14, and then is introduced into the cyclone 20.

In the cyclone, the gas and the solid, i.e., the coupling product gas 3and the catalyst 5, are separated. The coupling product gas 3 is mixedwith a cracking product gas 6 (which will be explained below) from thecracking reactor 30, and then the mixture is collected as a product gasmixture 7.

The catalyst 5 descends gravitationally from the cyclone dip leg 21, andthen is suspended in the cracking reactor 30 by a cracking feed gas 8containing a saturated hydrocarbon of C₂₊, which is injected in thecracking reactor 30 via the ring-shaped distributor 32. As the catalyst5 is fluidized in the cracking reactor 30, it makes close contact withthe cracking feed gas 8.

Then, a cracking reaction occurs so as to produce a cracking product gas6 which contains an unsaturated hydrocarbon. After the cracking productgas 6 is separated from the suspended catalyst by, for example, asecondary cyclone (which is not shown), the cracking product gas 6 ismixed with the coupling product gas 3, and then the mixture is collectedas the product gas mixture 7.

In the meantime, the separated suspended catalyst is allowed to returnto the cracking reactor 30.

The catalyst which is fluidized in the cracking reactor 30 is cooled bythe cracking reaction which is endothermic. The catalyst is furthermorecooled to a desirable temperature by having contact with the catalystcooling means 40. Then, the catalyst is withdrawn from a bottom portionof the cracking reactor 30, then carried from the catalyst withdrawaloutlet 33 by the catalyst feedback means 50, and then fed back to thecatalyst-bed formation section 11 of the coupling reactor 10 as acoupling catalyst for a following cycle.

According to the above method, since in the coupling reactor 10, thecatalyst is dispersed and suspended in the ascending stream which isformed by the coupling feed gas 1, a uniform conveying catalyst bed canbe formed. By introducing the gas containing oxygen 2 into the conveyingcatalyst bed, a coupling reaction occurs. Although the coupling reactionis exothermic, the continuous supply of coupling feed gas 1 and thecatalyst 5, which have relatively low temperatures, to the couplingreaction section 13, and the continuous introduction of a regulatedamount of the gas containing oxygen 2 from the gas introduction nozzle12, as well as the immediate discharge of the coupling product gas 3 andthe catalyst 5, which have reached high temperatures, to the outside ofthe reaction system, and the feedback of the separated catalyst 5 whichhas been cooled, maintains the temperature in the coupling reactor 10within a permissible range without abrupt increases, and thus hindersthe undesirable combustion reaction, improving the selectivity of thecoupling reaction.

Although the coupling feed gas 1 is a gas used for forming the ascendingstream according to the above embodiment, the method of the presentinvention is not limited to such an embodiment; for example, an inertgas such as nitrogen, or the gas containing oxygen 2 may be employed asthe gas for forming the ascending stream. There may be a case in whichan inert gas is advantageously used for controlling the temperature inthe coupling reactor 10.

Moreover, although the entire amount of the coupling feed gas 1 issupplied to the catalyst-bed formation section 11, and the entire amountof the gas containing oxygen 2 is introduced from the sole gasintroduction nozzle 12 according to the above embodiment, the method ofthe present invention is not limited to such an embodiment; for example,at least a part of the coupling feed gas 1 and at least a part of thegas containing oxygen 2 may be supplied from another nozzle (which isnot shown) which is provided at the coupling reaction section 13, or atleast a part of both gasses may be simultaneously supplied from a solegas introduction nozzle 12. In particular, a double-tube nozzle (whichis not shown) for introducing simultaneously at least a part of thecoupling feed gas 1 and at least a part of the gas containing oxygen 2may be advantageously used for controlling the temperature.

By bringing the catalyst, which is heated to a high temperature, andwhich is separated by the gas-solid separator (cyclone) 20, into contactwith the cracking feed gas 8 in the cracking reactor 30, the crackingproduct gas 6 which contains an unsaturated hydrocarbon is produced.Although the cracking reaction is endothermic, the heat which ispossessed by the catalyst 5 can be used for the reaction; therefore, thereaction proceeds without necessity of additional heat from outside,while the catalyst is cooled to a desirable temperature, and is fed backto the coupling reactor 10.

Since the cracking reaction is a reaction to produce an unsaturatedhydrocarbon from a saturated hydrocarbon of C₂₊, the concentration ofthe unsaturated hydrocarbon in the product gas mixture 7 increases bythis reaction. That is to say, by separating in advance the natural gasinto the coupling feed gas 1, which is rich in methane, and the crackingfeed gas 8, which is rich in the saturated hydrocarbon of C₂₊, andsupplying these gases to the respective reaction systems, the yield ofthe unsaturated hydrocarbon as a whole which is produced from thenatural gas is improved.

As the coupling feed gas 1, natural gas, or a methane-rich gas which isobtained by separating a gas which is rich in a saturated hydrocarbon ofC₂₊ from natural gas as in the above, may be advantageously used.However, it goes without saying that a methane-rich gas which isobtainable from another supply source can be also used.

As the gas containing oxygen 2, air can be used; however, in view of thereduction in the scale of the installation, the control of thetemperature, the prevention of the formation of nitrogen oxides, and theremoval of nitrogen, a gas in which nitrogen and other inert componentshave been removed as much as possible is preferable.

As the catalyst 5, a basic oxide having at least one element selectedfrom the group consisting of alkaline metals, alkaline earth metals, andrare earth elements, a carbonate salt having at least one of theseelements, or a compound oxide of at least two of these elements or of atleast one of these elements in combination with the other elements. Asexamples of the catalyst 5, SrO.La₂ O₃, BaO.La₂ O₃, BaO.CeO₂,NaMnO₄.MgO, Na₂ P₂ O₇.Mn.SiO₂, Li.Mn.B.MgO, and CaCO₃ can be cited.These are used in the forms of powder or granule so as to be distributedand suspended in the ascending stream in the coupling reactor 10 and inthe gas stream in the cracking reactor 30.

The catalyst 5 can be repeatedly used by separating it from the couplingproduct gas 3, then cooling it to about 750° C., and then feeding itback to the catalyst-bed formation section in the, coupling reactor 10.

As explained in the above, an extremely large amount of heat isgenerated in the coupling reactor 10. Especially in the vicinity of thegas introduction nozzle 12 for introducing the gas containing oxygen 2,the high concentration of the oxygen abruptly raises the temperature. Inorder to prevent this local overheating, the gas containing oxygen 2 maybe separately supplied, as explained in the above, through at least twolocations which are distant along the direction in which the gas streamascends. This is enabled by providing gas introduction nozzles forintroducing the gas containing oxygen at two locations (not shown) ormore along the longitudinal direction of the coupling reactor 10. Moreprecise control at a uniform temperature in the coupling reactor becomespossible by making the amount of the gas containing oxygen which isintroduced through each gas introduction nozzle individually adjustable.

Alternatively, in order to prevent the local temperature from risingabruptly in the coupling reactor 10, at least a part of the couplingfeed gas 1 may be supplied to at least one location in the ascendingstream. This can be implemented by providing one or more locations onthe coupling reaction section 13 with gas introduction nozzles (notshown) for introducing the coupling feed gas. In particular, bycoaxially supplying the coupling feed gas and the gas containing oxygenusing a double-tube nozzle, a gas in the zone where the temperaturebecomes highest can be diffused by a cold gas stream; therefore, this iseffective in making the temperature distribution in the couplingreaction section 13 uniform. It is also possible and effective toprovide a plurality of gas introduction nozzles, that is, gasintroduction nozzles for introducing the gas containing oxygen and gasintroduction nozzles for introducing the coupling feed gas.

In each case where these methods and apparatuses are employed, it ispreferable, in order to disperse the catalyst, that at least a part ofthe coupling feed gas is introduced at the bottom of the couplingreactor 10.

A suitable ratio of the coupling feed gas and the gas containing oxygenwhich are supplied to the coupling step depends on the amount of themethane and oxygen contained respectively. It is preferable that themolar ratio of oxygen/methane is adjusted in the range between 0.05 and0.5.

The reaction for producing ethylene by coupling proceeds basically inaccordance with the following formula (2):

    2CH.sub.4 +O.sub.2 →C.sub.2 H.sub.4 +2H.sub.2 O     (2)

In addition, in parallel with the above reaction, ethane is produced inaccordance with the following formula (3):

    2CH.sub.4 +1/2O.sub.2 →C.sub.2 H.sub.6 +H.sub.2 O   (3)

The above reactions proceed at high selectivities when the temperaturein the coupling reaction section 13 is in the range betweenapproximately 750° C. and 900° C. On the other hand, in this temperaturerange, in parallel with these reactions, a combustion reaction of thecoupling feed gas containing methane also occurs. When the amount ofoxygen which is supplied under the above condition is less than 0.05times that of the methane, the oxygen is in short supply; the conversionof methane decreases, and thus the yields of ethylene and ethane lower.Moreover, since the yield per pass lowers, the scale of the installationbecomes too large. On the other hand, when the amount of oxygen exceeds0.5 times that of the methane, the excessive oxygen allows thecombustion to become the major reaction, and thus the selectivities ofthe reactions of formulae (2) and (3) for producing ethylene and methanedecline.

It is preferable that the amount of the catalyst which is used in thecoupling step is adjusted so that the concentration of the catalystwhich is dispersed in the coupling reaction section 13 is maintained inthe range between 0.5% to 5% by volume. As well as the catalystcatalytically promoting the reactions of the above formulae (2) and (3),it fulfills a role in a temperature regulating effect in which thetemperature in the coupling reaction section 13 is maintained in asuitable range for the reaction. When the concentration of the catalystis less than 0.5% by volume, the catalyst effect and the temperatureregulating effect are insufficient. On the other hand, when theconcentration of the catalyst exceeds 5% by weight, it becomes difficultto keep the catalyst dispersed and suspended in a stable condition. Inview of the above, it is more preferable that the concentration of thecatalyst is adjusted in the range between 1% and 3% by volume.

It is desirable, for the purpose of maintaining a high selectivity and ahigh conversion, that the temperature in the coupling reaction section13 is adjusted to be constant in the range between 750° C. and 900° C.In particular, the reaction proceeds most smoothly at a temperaturebetween 800° C. and 900° C. Maintenance of the temperature can becarried out by regulating the flow rates of the coupling feed gas 1 andthe gas containing oxygen 2 as well as the amount of the circulatingcatalyst and the concentration of the catalyst; the maintenance of thetemperature also depends on the temperature of the catalyst at the timeof introduction. It is preferable that the catalyst to be supplied tothe coupling reaction section 13 be cooled to approximately 750° C.

It is preferable that the reaction pressure in the coupling step bemaintained in a range between atmospheric pressure and 10 kg/cm² G. Whenthe reaction pressure is less than atmospheric pressure, a specificapparatus is required for decompression, and no special advantage can beobtained. When the reaction pressure exceeds 10 kg/cm² G, the combustionreaction becomes dominant, and the selectivities of the reactions forproducing ethylene and methane decrease.

Since the catalyst from the coupling reactor 10 is at a hightemperature, it is preferable to make an effective use of this heat. Inview of this, in the cracking reactor 30, cracking of a saturatedhydrocarbon of C₂₊ is carried out using this catalyst at a hightemperature as a heat transfer medium.

As in the above, as the cracking feed gas 8, a gas which is rich in ahydrocarbon of C₂₊ which is separated from natural gas is preferablyused. Among such gases which are rich in a hydrocarbon of C₂₊, forexample, LPG is known, which mainly contains methane, ethane, propane,butane, and the like. However, a gas rich in a hydrocarbon of C₂₊ whichis obtained from another source can be also used, if necessary, as apart or an entirety of the cracking feed gas 8.

The reaction for producing ethylene by cracking ethane proceeds inaccordance with the following formula (4):

    C.sub.2 H.sub.6 →C.sub.2 H.sub.4 +H.sub.2           (4)

The above reaction is endothermic, and it proceeds by bringing thecracking feed gas 8 into contact with the catalyst which is heated toapproximately 850° C. Since this reaction deprives the catalyst of heat,the temperature of the catalyst normally falls by a decrement ofapproximately between 50° C. and 100° C.

The cracking reactor 30 is a fluidized-bed reactor, into which thecracking feed gas 8 is introduced, for example, via a ring-shapeddistributor 32 which is provided in the vicinity of a bottom portion.The catalyst 5, which is at a high temperature, is continuously suppliedto the cracking reactor 30, where the catalyst 5 and the cracking feedgas 8 are brought into contact in a fluidizing state. When thetemperature of the catalyst which is withdrawn from the bottom of thecracking reactor 30 is higher than approximately 750° C., the catalystis cooled to a predetermined temperature by using the catalyst coolingmeans 40 as the need arises.

Since the cracking product gas 6 which is obtained as the result of thecracking contains an unsaturated hydrocarbon in a relatively highconcentration, the cracking product gas 6 is mixed with the couplingproduct gas 3, so as to be collected as the product gas mixture 7, whichis rich in the unsaturated hydrocarbon.

The thus-collected product gas mixture 7 contains reaction products suchas ethylene, ethane, saturated and unsaturated hydrocarbons of C₃₊, H₂O, and H₂, as well as unreacted methane and other hydrocarbons from thefeeds, and CO and CO₂ concurrently produced as the result of thecombustion reaction. It is as a matter of course that one of thepurposes of the present invention is to produce the product gas mixture7 which is rich in the C₂₊ hydrocarbons; in addition, in order toenhance the total yield of the C₂₊ hydrocarbons, it is desired that asmuch residual gas as possible remaining after collecting the product gasmixture is fed back and reused as a portion of the coupling feed gas 1.

An embodiment of a method for reusing the residual gas is shown in FIG.2. In FIG. 2, a product gas mixture 7 which is collected from a couplingapparatus 60 is sent to a C₂₊ separation-collection apparatus 70, wherea C₂₊ hydrocarbon 71 is separated and collected, and the residual gas 72mainly containing unreacted CH₄, H₂, CO, CO₂, and N₂ is sent to thefeedback system.

Before the product gas mixture 7 which is withdrawn from the couplingapparatus 60 is sent to the C₂₊ separation-collection apparatus 70, itmay be introduced into a polymerization apparatus 100 for polymerizingan unsaturated hydrocarbon in the product gas mixture. Then, apolymerization product gas 7r is sent to the C₂₊ separation-collectionapparatus 70. Contents produced as the result of the polymerization inthe polymerization apparatus 100 are collected from a line of the C₂₊hydrocarbon 71 as gasoline and kerosene fractions.

A feedback system to which the residual gas 72 is sent is constructedmainly from a decarbonation apparatus 80 and a methanation apparatus 90.

The residual gas 72 is split into a branch gas 74 and a bypass gas 75 bya distributing valve 73. The branch gas 74 is introduced into thedecarbonation apparatus 80 in which CO₂ contained in the branch gas 74is removed. A decarbonated gas 81 which is obtained after the removal ofCO₂ is brought into confluence with the bypass gas 75. The thus-obtainedconfluence gas 82 is introduced into the methanation apparatus 90 so asto be methanated. At least a part of the thus-obtained methanated gas 91is fed back to the coupling apparatus 60 as at least a part of acoupling feed gas 1.

By removing CO₂ from a part of the residual gas 72 according to theabove method, the molar ratios of carbon oxides, consisting of CO andCO₂, and H₂, which are in the residual gas 72, are adjusted tostoichiometric ratios which are suitable for the methanation reaction.Accordingly, the obtained methanated gas 91 does not contain CO and CO₂which are unnecessary for the coupling reaction, and thus the methanatedgas 91 can be fed back and used as the coupling feed gas 1 which is richin methane. Moreover, since only the branch gas 74 is decarbonatedinstead of the entire residual gas, the decarbonation apparatus can besmall, and which consumes little energy for the decarbonation.

Molar ratios of CO, CO₂, and H₂ in the coupling product gas which isobtained from the coupling apparatus 60 according to the aboveembodiment is approximately as follows:

CO: 0.3˜0.6% by mole

CO₂ : 1.5˜3.0% by mole

H₂ : 2.0˜6.0% by mole

In addition, the methanation reaction proceeds in accordance with thefollowing formulae (5) and (6):

    CO+3H.sub.2 →CH.sub.4 +H.sub.2 O                    (5)

    CO.sub.2 +3H.sub.2 →CH.sub.4 +2H.sub.2 O            (6)

Therefore, when the methanation is conducted using CO, CO₂, and H₂ inthe coupling product gas without adjusting compositions, carbon oxides(CO and CO₂) are excessive with respect to H₂. When this is the case,the entire amount of H₂ is not consumed in the reaction with CO alone;therefore, by removing a part of CO₂ from the residual gas 72, the molarratios of carbon oxides and H₂ can be adjusted to stoichiometric ratioswhich are suitable for the methanation reaction.

The splitting ratios of the branch gas 74 and the bypass gas 75 splitfrom the residual gas 72 can be controlled by measuring the molarcontents of each of the above components in the residual gas 72 and theconfluence gas 82, and then adjusting distribution proportions with thedistributing valve 73 so as to maintain the component ratios in theconfluence gas 82 within a range of values which are most suitable forthe methanation.

Another embodiment of a method for feeding back and reusing the residualgas is shown in FIG. 3. In FIG. 3, a residual gas 72, which is withdrawnfrom a C₂₊ separation-collection apparatus 70, is sent firstly to amethanation apparatus 90, and secondly to a feedback system comprising adecarbonation apparatus 80.

When the above is the case, the residual gas 72 is first introduced intothe methanation apparatus 90 so as to be methanated. The thus-obtainedmethanated gas 91 contains an excessive amount of CO₂ in addition toproduced methane. Therefore, the methanated gas 91 is then introduced inthe decarbonation apparatus 80, in which the surplus CO₂ is removed. Atleast a part of the thus-obtained decarbonated gas 81 is fed back as atleast a part of a coupling feed gas 1 to a coupling apparatus 60.

According to the above method, since the surplus CO₂ is not preliminaryremoved from the residual gas 72, the methanation is carried out under acondition in which the amount of CO₂ is in excess. When the amount ofCO₂ is in excess in the reaction of CO₂ and H₂ as expressed in theformula (6), the equilibrium of the reaction inclines toward theproduction of methane, and thus the amount of the unreacted residualhydrogen decreases. If the residual hydrogen is fed back to the couplingapparatus 60, oxidation of the residual hydrogen occurs in the couplingreactor 10, which tends to cause unusual temperature rising.

Therefore, the method according to the above embodiment, in which theamount of the residual hydrogen is reduced, is advantageous incontrolling the temperature in the coupling reactor 10. Moreover, sinceacidic impurities which are produced as by-products in the methanationapparatus are removed by the decarbonation apparatus 80 which carriesout the following process, the method according to the above embodimentis also advantageous in view of enhancement of the partial pressure ofmethane by way of removing as much surplus content as possible from thecoupling feed gas.

In each embodiment of the above feedback systems, the decarbonation stepcarried out in the decarbonation apparatus 80 may usually employs amethod in which an acidic gas is absorbed in an alkaline absorbent, andthen, by heat stripping, the acidic gas is stripped from the liquidwhich has absorbed the acidic gas. In this method, the separated CO₂ iscollected via a line 83. On the other hand, in the methanation apparatus90, a gas phase reaction using a methanation catalyst is employedusually.

Natural gas sometimes contain CO₂ or H₂ S, which are acidic, and inparticular, H₂ S adversely affects the reaction and the product;therefore, when applying the method of the present invention tooxidative coupling of natural gas in such a case, CO₂ and H₂ S must beremoved before the natural gas is introduced into the coupling apparatus60. The removal can be carried out using an exclusive installation;however, by introducing natural gas via a line 84 into the decarbonationapparatus 80 as shown in FIGS. 2 and 3, the decarbonation apparatusremoves H₂ S as well as CO₂, and thus the coupling feed gas 1 can befree of sulfur.

Moreover, as shown in FIGS. 4 and 5, it is possible that the sulfur-freenatural gas can be introduced into an LNG separation apparatus 110,which is provided in a feedback system, in which a C₂₊ hydrocarboncontained in the natural gas is separated out, and then the separatedC₂₊ hydrocarbon is supplied to a cracking reactor 30 in a crackingapparatus 60 as a cracking feed gas 8, while the remaining gas (a line82r in FIG. 4, or a line 81r in FIG. 5) is introduced into the feed backsystem as a coupling feed gas. In short, FIG. 4 illustrates anembodiment of an arrangement of the LNG separation apparatus 110 in thecase where the decarbonation step conducted in the decarbonationapparatus 80 as shown in FIG. 2 precedes the methanation step conductedin the methanation apparatus 90, wherein LNG separation apparatus 110 isarranged in the line of confluence gas 82. On the other hand, FIG. 5illustrates another embodiment of an arrangement of the LNG separationapparatus 110 in the case where the methanation step conducted in themethanation apparatus 90 as shown in FIG. 3 precedes the decarbonationstep conducted in the decarbonation apparatus 80. When the latter is thecase, the LNG separation apparatus 110 is inserted into a decarbonatedgas line 81 between the decarbonation apparatus 80 and couplingapparatus 60.

Although there may be cases where natural gas contains some or no acidicgas depending on the place of the production, the above method in anycase can effect acquisition of both the coupling feed gas 1 to besupplied to the coupling apparatus 60 and the cracking feed gas 8, fromthe gas line.

In the feedback system of the residual gas as shown in FIGS. 2 and 3,the continuous feedback of the gas results in the gradual accumulationof gases which do not participate in the reaction, such as N₂. In orderto remove such gases, it is preferable, with respect to the embodimentsshown in FIGS. 2 and 3, that at least a part of the gas in the feedbacksystem be continuously or intermittently purged via at least one of apurge line 76 branching from the residual gas 72, a purge line 85branching from an intermediate portion between the decarbonationapparatus 80 and the methanation apparatus 90, and a purge line 92branching from an input line for the coupling apparatus 60.

The coupling product gas 3, the product gas mixture 7, or the C₂₊hydrocarbon 71, which are components obtained according to the method ofthe present invention as explained in detail in the above, can be usedas: a fuel, after each of these components are separated, or as amixture; a material for synthesizing gasoline or kerosene fractionsafter the components are polymerized; or a material for other variousorganic syntheses. The thus-obtained products from the C₂₊ hydrocarboncan be easily liquefied, and thus are convenient for storage andtransportation.

EXAMPLES

Example 1

An experiment simulating a plant which can process natural gas 500000Nm³ /day was carried out by using the coupling apparatus 60 as shown inFIG. 1. As for feeds, separation of a C₂₊ hydrocarbon from natural gaswas conducted; a fraction which was rich in methane was used as acoupling feed gas 1, while a fraction which was rich in the C₂₊hydrocarbon was used as a cracking feed gas 8. As a catalyst, CaO--La₂O₃ --LiO--SiO₂ catalyst was used.

With regard to each of the coupling feed gas 1 (Line 1), the gascontaining oxygen 2 (Line 2), the cracking feed gas 8 (Line 8), and theproduct gas mixture 7 (Line 7), the amount of each component in the unitof % by mole, the flow rate in Nm³ /day, the temperature in °C., and thepressure in kg/cm² G are shown in Table 1.

                  TABLE 1                                                         ______________________________________                                                  1        2          8      7                                                  Coupling Gas        Cracking                                                                             Product                                            feed     containing feed   gas                                      Line      gas      oxygen     gas    mixture                                  ______________________________________                                        H.sub.2 O 2.14     0.00       0.01   13.55                                    CO.sub.2  0.14     0.00       4.31   2.24                                     CO        0.00     0.00       0.09   0.48                                     H.sub.2   0.59     0.00       0.12   4.30                                     N.sub.2   12.38    0.50       0.91   9.69                                     O.sub.2   0.00     99.50      0.00   0.00                                     C.sub.1   84.12    0.00       58.62  59.71                                    C.sub.2 ═                                                                           0.16     0.00       1.81   6.80                                     C.sub.2   0.46     0.00       20.13  1.93                                     C.sub.3 ═                                                                           0.00     0.00       0.41   0.59                                     C.sub.3   0.01     0.00       5.60   0.24                                     C.sub.4 ═                                                                           0.00     0.00       1.96   0.23                                     i-C.sub.4 0.00     0.00       2.90   0.11                                     n-C.sub.4 0.00     0.00       2.86   0.11                                     C.sub.5 ˜C.sub.9                                                                  0.00     0.00       0.21   0.01                                     C.sub.10           0.00       0.06   0.01                                     Flow rate 43.5     4.4        6.5    56.3                                     (Nm.sup.3 /day)                                                               Temperature                                                                             305.8    20.0       490.0  800.0                                    (°C.)                                                                  Pressure  4.6      4.6        4.5    4.2                                      (kg/cm.sup.2 G)                                                               ______________________________________                                    

The results from the experiment according to Example 1 indicate that theproduced amount of ethylene in the product gas mixture 7 was more thanthat of carbon oxides. This is believed to have resulted from theselectivity of the coupling reaction exceeding that of the oxidationreaction, since the highest temperature in the coupling reactor 10,which was detected as the temperature of the product gas mixture 7, wasadjusted to 800° C. Moreover, it can be seen that the conversion ofethylene according to this method was extremely high, as the ratio ofthe total mole fraction of ethylene contained in the coupling feed gas 1and the cracking feed gas 8 to the mole fraction of ethylene containedin the product gas mixture 7 was 0.1873 (feeds):3.828 (product gasmixture), i.e., 1:20.4.

Example 2

An experiment simulating a plant which can process natural gas 500000Nm³ /day was carried out by connecting the coupling apparatus 60 inExample 1 with a C₂₊ separation-collection apparatus 70, a decarbonationapparatus 80, and a methanation apparatus 90 in successive order asshown in FIG. 2 so as to form a feedback system.

Thus, the product gas mixture 7 obtained by Example 1 was separated inthe C₂₊ separation-collection apparatus 70 into a C₂₊ hydrocarbon 71 andthe residual gas 72, the C₂₊ hydrocarbon 71 was collected, and a part ofthe residual gas 72 was branched to the decarbonation apparatus 80 to bedecarbonated, and the decarbonated gas 81 was brought into confluencewith a bypass gas 75. From the confluence gas 82, as shown in FIG. 4, afraction 8, which was rich in C₂₊, was separated via an LNG separationapparatus 110; a fraction 82r, which was rich in methane, was methanatedin the methanation apparatus 90; and the methanated gas 91 was fed backto the coupling apparatus 60. The fraction which was rich in C₂₊ wasused as the cracking feed gas 8.

In the meantime, the feed natural gas was introduced in thedecarbonation apparatus 80 via line 84 so as to remove H₂ S. Inaddition, the confluence gas 82 was purged continuously at a constantrate from a purge line 85. The distribution ratio of the branch gas 74to the bypass gas 75 was arranged to be approximately 75:25.

The load (in kg.mol/hr) in each apparatus and line is shown in FIG. 2.

Comparative Example 1

An operation according to Example 2 was repeated except that the bypassline 75 was closed so that the entire amount of the residual gas 72 wassupplied from the branch gas line 74 to the decarbonation apparatus 80,and shortage in CO₂ in the methanation apparatus 90 was supplied fromthe outside, via a line (which is not shown), to the methanationapparatus 90 according to a conventional method.

The load (in kg.mol/hr) in each apparatus and line is shown in Table 2.

                  TABLE 2                                                         ______________________________________                                                                         Comparative                                                         Example 2 Example 1                                    Step        Line No.   (kg.mol/hr)                                                                             (kg.mol/hr)                                  ______________________________________                                        Feed gas    84          930.1     930.1                                       Gas containing oxygen                                                                      2          532.0     532.0                                       Bypass gas  75         1563.3      0.0                                        Branch gas  74         5657.7    7221.0                                       Decarbonation step                                                                        80         5477.3    7002.5                                       Supplementary CO.sub.2   0.0      38.1                                        Purging gas 85          132.3     132.3                                       Confluence gas                                                                            82         6908.3    6908.3                                       Methanated gas                                                                            91         6160.7    6160.7                                       ______________________________________                                    

With regard to each of Example 2 and Comparative Example 1, the gassupply amount and CO₂ processing amount (both amounts are expressed inkg.mol/hr) in the decarbonation apparatus 80, and the thermal load (inkcal/hr) in a stripping column (not shown) for CO₂ stripping are shownin Table 3.

                  TABLE 3                                                         ______________________________________                                                                 Comparative                                                            Example 2                                                                            Example 1                                            ______________________________________                                        Amount of gas supplied                                                                            5657.7   7221.0                                           (kg.mol/hr)                                                                   Amount of CO.sub.2 processed                                                                       180.4    218.5                                           (kg.mol/hr)                                                                   Thermal load in stripping column                                                                  5.6 × 10.sup.6                                                                   6.9 × 10.sup.6                             (kcal/hr)                                                                     ______________________________________                                    

The above results indicate that, in comparison with the conventionalmethod according to Comparative Example 1 in which the residual gas 72was not branched, the method of Example 2, in which the residual gas 72was branched, reduced 22% of the amount of gas supplied, 17% of theamount of CO₂ processed and 19% of the thermal load. However, there wasno difference between Example 2 and comparative Example 1 in the loadsin the methanation apparatus 90 and coupling apparatus 60.

Example 3

An experiment simulating a plant which can process natural gas 500000Nm³ /day was carried out by connecting the coupling apparatus 60 inExample 1 with a C₂₊ separation-collection apparatus 70, a methanationapparatus 90, and a decarbonation apparatus 80 in successive order asshown in FIG. 3 so as to form a feedback system.

Thus, the product gas mixture 7 obtained by Example 1 was separated inthe C₂₊ separation-collection apparatus 70 into a C₂₊ hydrocarbon 71 andthe residual gas 72, the C₂₊ hydrocarbon 71 was collected, and theentire amount of the residual gas 72 was initially methanated in themethanation apparatus 90, the methanated gas 91 was decarbonated in thedecarbonation apparatus 80, the decarbonated gas 81 was processed in theLNG separation apparatus 110, and the fraction 81r which was rich inmethane was fed back to the coupling apparatus 60.

In the meantime, the feed natural gas was introduced in thedecarbonation apparatus 80 via line 84 so as to remove H₂ S. Inaddition, the confluence gas 85 was purged continuously at apredetermined rate from a purge line 85.

With regard to each of Example 3 and Comparative Example 1, the H₂conversion (%) and CO₂ consumption (kg.mol/hr) are shown in Table 4. Itis noted that in Comparative Example 1, CO₂ in stoichiometric ratiocorresponding to hydrogen in the feed gas was added to the methanationapparatus 90. In addition, in each of Example 3 and Comparative Example1, approximately the entire amount of CO in the feed gas was methanated.

                  TABLE 4                                                         ______________________________________                                                                Comparative                                                            Example 3                                                                            Example 1                                             ______________________________________                                        H.sub.2 conversion (%)                                                                           95.0     89.3                                              CO.sub.2 consumption (kg.mol/hr)                                                                 32.3     30.5                                              ______________________________________                                    

The above results indicate that the equilibrium of methanation shiftedtoward the production of methane according to the method of Example 3 inwhich an excessive amount of carbon oxides were supplied to themethanation apparatus 90; thus, both the H₂ conversion and CO₂consumption were improved in comparison with Comparative Example 1.These results indicate that amounts of H₂, CO₂, and H₂ O, which wouldhinder the coupling reaction, in the feed gas 1 which was supplied tothe coupling apparatus was reduced.

By comparing Example 2 with Example 3, it can be concluded that, whenthe method is designed to reduce the load to the decarbonation apparatus80 and to reduce the installation scale and energy consumption thereof,the method of Example 2 should be preferably adopted; and, when themethod is designed to reduce as much as possible components other thanmethane in the feed gas 1 to be supplied to the coupling apparatus 60 soas to improve the reaction ratio of the coupling reaction, the method ofExample 3 should be preferably adopted.

What is claimed is:
 1. An apparatus for oxidative coupling of methanewhereby reaction gases, which include a coupling feed gas containingmethane and a gas containing oxygen, are allowed to react in thepresence of a catalyst, and a coupling product gas containing anunsaturated hydrocarbon is produced;said apparatus for oxidativecoupling of methane comprising a coupling reactor in a shape of avertical pipe, a gas-solid separator, and a catalyst feedback means,which are connected in a loop; said coupling reactor comprising acatalyst-bed formation section, which is provided at a bottom portion ofsaid coupling reactor, for forming a conveying catalyst bed which isformed by an ascending stream which contains said catalyst, a gasintroduction nozzle for supplying at least a part of at least one ofsaid reaction gases to said conveying catalyst bed, a coupling reactionsection for producing a coupling product gas by allowing methane andoxygen to react in the presence of said catalyst, and acoupling-product-gas withdrawl outlet, which is provided at a topportion of said coupling reactor, for withdrawing said coupling productgas together with said catalyst; said gas-solid separator being providedfor separating said coupling product gas and said catalyst; and saidcatalyst feedback means being provided for feeding back said catalyst,which is separated in said gas-solid separator, to said catalyst-bedformation section of said coupling reactor wherein said apparatus foroxidative coupling of methane further comprises between said gas-solidseparator and said catalyst feedback means, a catalyst cooling means forcooling said catalyst.
 2. An apparatus for oxidative coupling of methaneas in claim 1, wherein said gas introduction nozzle is a double-tubenozzle for simultaneously introducing at least a part of said couplingfeed gas and at least a part of said gas containing oxygen.
 3. Anapparatus for oxidative coupling of methane as in claim 1, wherein saidgas-solid separator is a cyclone.
 4. An apparatus for oxidative couplingof methane as in claim 1, wherein said apparatus for oxidative couplingof methane further comprises, between said gas-solid separator and saidcatalyst feedback means, a cracking reactor for forming a fluidized bedfrom said catalyst which is separated in said gas-solid separator and acracking feed gas which contains a saturated hydrocarbon having a carbonnumber of at least 2, so as to bring said catalyst and said crackingfeed gas into contact with each other, and for allowing a crackingreaction to occur so as to produce a cracking product gas which containsan unsaturated hydrocarbon.